The present invention relates to a system for measuring very minute time differences--of the order of microseconds or less--between the occurrences of an electrical signal at two spaced points in a system. A problem of this sort might occur, for example, in the measurement of the time difference in reception of an electromagnetic wave at two spaced, fixed antenna locations.
Such a technique is useful in a direction finder for sferics. A sferic is a transient electromagnetic wave caused by an atmospheric disturbance. The electromagnetic wave is detected by an antenna at each location. By using three or more antennas at known locations (such as to form two baselines at a non-zero angle to each other) two angles of incidence (azimuth and elevation), of the sferic wave upon the receiving location may be calculated from the time difference measurements. Such an instrument can provide information about the discharge processes within a storm.
The present invention will be illustrated with two received signals, but the method and apparatus disclosed can easily be extended to include three or more received signals if desired.
Briefly, prior time-interval measurement systems have required very wide bandwidths (a disadvantage when the signals arrive through antennas in the presence of interfering signals) and expensive and complicated circuitry and components for measuring very small intervals (of the order of 10 to 100 nanoseconds) directly with high resolution.
Each receiver in the present invention is provided with a coaxial resonator for converting an incident transient signal to a damped sinusoid so that the advantages of phase techniques may be exploited. The resonators are carefully matched in frequency and "Q" (quality factor or damping coefficient) so that their outputs remain phase-stable over several hundred cycles of the lightly-damped sinusoidal output. One of the advantages of using coaxial resonators is that this matching can be obtained.
The resonators, upon reception of a transient signal, begin to oscillate in timed relation with the reception of the received signal. Each resonator output signal is mixed with the output signal of a common local oscillator; the respective mixed signals are amplified by intermediate frequency (IF) amplifiers.
The phase difference of the output signals of the IF amplifiers is representative of the difference in time of reception of the signals at the two locations, but on an expanded time scale. With the present system, for example, a time scale magnification of 100 is easily accomplished.
The phase difference between the two signals in the IF amplifiers is detected and used to gate the output signal of a time mark oscillator into a counter circuit. Thus, the accumulated contents of the counter is representative of the time difference of reception. Circuitry is also provided for indicating the polarity of the phase shift (i.e., whether the phase shift is leading or lagging.)
The present invention thus provides a very accurate and reliable system for measuring very small time differences between received signals. Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing.